Testing visible ozone injury within a Light Exposed Sampling Site as a proxy for ozone risk assessment for European forests

Biologically meaningful and cost-efective indicators are needed for assessing and monitoring the impacts of tropospheric ozone (O3) on vegetation and are required in Europe by the National Emission Ceilings Directive (2016). However, a clear understanding on the best suited indicators is missing. The MOTTLES (MOnitoring ozone injury for seTTing new critical LEvelS) project set up a new generation network for O3 monitoring in forest plots in order to: 1) estimate the stomatal O3 fuxes (Phytotoxic Ozone Dose above a threshold Y of uptake, PODY); and 2) collect visible foliar O3 injury, both within the forest plot (ITP) and along the Light Exposed Sampling Site (LESS) along the forest edge. Nine forest sites at high O3 risk were selected across Italy over 2017-2019 and signifcant correlations (p<0.05) were found between the percentage of symptomatic plant species within the LESS, and POD1 (PODY, with Y=1 nmol O3 m-2 s-1) calculated for mixed forest species (r=0.53) and with the occurrence and severity of visible foliar O3 injury on the dominant species in the plots (r=0.65). A generic fux-based critical level for mixed forest species was derived within the LESS and it was recommended using 11 mmol m-2 POD1 as the critical level for forest protection against O3 injury, similar to the critical level obtained in the ITP (12 mmol m-2 POD1). It was concluded that the frequency of symptomatic plant species within a LESS is a suitable and efective plant-response indicator of phytotoxic O3 levels in forest monitoring. LESS is a non-destructive, less complex and less time-consuming approach compared to the ITP for monitoring foliar O3 injury in the long term. Assessing visible foliar O3 injury in the ITP might only underestimate the O3 risk assessment at ndividual sites. These results are biologically meaningful and useful to monitoring experts and environmental policy makers.